Capacitance variation detection circuit, touch screen and touch detection method

ABSTRACT

The present disclosure relates to a capacitance variation detection circuit, a touch screen and a touch detection method. The method includes: connecting one terminal of a detection capacitor and one terminal of a denoising capacitor to a first power simultaneously, and connecting the other terminal of the detection capacitor and the other terminal of a denoising capacitor to a reference ground simultaneously; disconnecting the detection capacitor from the first power source and connecting it to the negative input terminal of the operation amplifier, switching the one terminal of the denosing capacitor from being connected to the first power source to being connected to the reference ground, and connecting the other terminal of the denoising capacitor to the negative input terminal of the operational amplifier; and acquiring an output result of the operational amplifier, and judging whether there is a touch operation according to the output result.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of international applicationNo. PCT/CN2016/103379, filed on Oct. 26, 2016, which is incorporatedherein by reference in its entirety.

TECHNICAL FIELD

Embodiments of the present disclosure relate to the technical field oftouch detection, and in particular, relate to a capacitance variationdetection circuit, a touch screen and a touch detection method.

BACKGROUND

In application of touch detection on a touch screen or fingerprintidentification, generally a touch operation or a fingerprint is detectedby detecting capacitance variation. In a capacitance variation detectioncircuit, a circuit structure in the prior art is as illustrated in FIG.1 and FIG. 2. In the state as illustrated in FIG. 1, switches S1, S3 andS5 are turned on and a detection capacitor Ctp is charged, chargesstored in capacitors C1 and C2 are released, and other switches are allturned off. In the state as illustrated in FIG. 2, switches S2 and S4are turned on, the other switches are turned off, charges stored in thedetection capacitor Ctp are re-allocated, and according to the law ofconservation of charge, the following equation is obtained:

Q=C _(tp) V _(pwr) =C _(tp) V _(ref) +C ₁ V _(ref) +C ₂(V _(ref) −V_(out))

An output after N times of sampling is:

$V_{out} = {V_{ref} - {N\frac{{C_{tp}\left( {V_{pwr} - V_{ref}} \right)} - {C_{1}V_{ref}}}{C_{2}}}}$

When the capacitance of the detection capacitor Ctp varies due to atouch press, a fingerprint or the like, the variation of the detectioncapacitor Ctp may be detected according to the output of the circuit.Ctp includes a basic value before variation, and a variation, which arerespectively Cbg and Csig. After N times of sampling, the followingequation is obtained:

$V_{out} = {V_{ref} - {N\frac{{C_{bg}\left( {V_{pwr} - V_{ref}} \right)} - {C_{1}V_{ref}}}{C_{2}}} - {N\frac{C_{sig}\left( {V_{pwr} - V_{ref}} \right)}{C_{2}}}}$

It is apparent that since C1 is added, a useless charge variation causedby capacitance basic value Cbg is offset, and the variation portion maybe only analyzed with respect to an output result.

However, during practice of the present disclosure, the inventors haveidentified that the above process has a problem:

Since Vpwr includes noise which comes from a driving circuit generatingVpwr, and other interference which is originated from a mobile orwearable device, including but not limited to interference from a screendriving signal or interference from a charger or interference from otherparts of the chip. Therefore, after N times of sampling and integration,the noise and interference may also be increased with the integration.As a result, the output signal has an excessively low signal-to-noiseratio, which reduces the accuracy of touch detection or fingerprintidentification.

SUMMARY

In view of the above, embodiments of the present disclosure provide acapacitance variation detection circuit, a touch screen and a touchdetection method, to solve the problem the accuracy of touch detectionor fingerprint identification is lowered due to noise and interference.By reducing or eliminating impacts caused by noise of a driving voltageand other interference signals superimposed on the driving voltage ontoan output of an integrating circuit, a signal-to-noise ratio of usefulsignals is improved, design requirements imposed by the circuit ontoother modules are lowered, and overall power consumption of a chip isreduced.

In a first aspect, embodiments of the present disclosure provide acapacitance variation detection circuit, configured to detect acapacitance variation of a detection capacitor, one terminal of thedetection capacitor being connected to a reference ground and the otherterminal of the detection capacitor being connected to a first powersource. The detection circuit includes:

an operational amplifier, a denoising capacitor, an integratingcapacitor and a plurality of switches;

where a positive input terminal of the operational amplifier isconnected to a second power source, and a negative input terminal of theoperational amplifier is connected to the other terminal of thedetection capacitor via a first switch;

where one terminal of the denoising capacitor is connected to the otherterminal of the detection capacitor via a second switch and is connectedto the reference ground via a third switch, and the other terminal ofthe denoising capacitor is connected to the first power source via afourth switch and is connected to the reference ground via a fifthswitch; and

where two terminals of the integrating capacitor are connected via asixth switch.

Further, the first power source is a driving voltage source and adriving current source.

Further, the denoising capacitor is a capacitance-adjustable capacitor.

Further, a first resistor is connected between the second switch and thenegative input terminal of the operation amplifier.

Further, a second resistor is connected between the second power sourceand the positive input terminal of the operation amplifier.

In a second aspect, embodiments of the present disclosure provide atouch screen. The touch screen includes the above capacitance variationdetection circuit.

In a third aspect, embodiments of the present disclosure provide a touchdetection method. The method includes:

at a first stage, connecting one terminal of a detection capacitor andone terminal of a denoising capacitor to a first power simultaneously,and connecting the other terminal of the detection capacitor and theother terminal of a denoising capacitor to a reference groundsimultaneously;

at a second stage, disconnecting the detection capacitor from the firstpower source, connecting the detection capacitor to the negative inputterminal of the operation amplifier, switching the one terminal of thedenosing capacitor from being connected to the first power source at thefirst stage to being connected to the reference ground, and connectingthe other terminal of the denoising capacitor to the negative inputterminal of the operational amplifier; and

acquiring an output result of the operational amplifier, and judgingwhether there is a touch operation according to the output result.

Further, the acquiring an output result of the operational amplifierincludes: repeatedly performing the first stage and the second stageaccording to a predetermined execution count to obtain the outputresult.

Further, the predetermined execution count varies with differentdetection capacitors.

Further, before the first stage, a switch between two electrode platesof the integrating capacitor is turned on to discharge the integratingcapacitor, and upon completion of discharge, the switch between the twoelectrode plates of the integrating capacitor is turned off again.

Further, a capacitance of the denoising capacitor is adjusted accordingto a capacitance of the detection capacitor.

With the capacitance variation detection circuit, the touch screen andthe touch detection method according to the embodiments of the presentdisclosure, a denoising capacitor samples noise and interference causedto a detection capacitor, and impacts caused by the noise andinterference are eliminated by means of charge re-allocation.Specifically, while the detection capacitor is being charged, thedenoising capacitor is connected to the same power source, andtherefore, the noise and interference loaded onto the detectioncapacitor and the denoising capacitor have the same phase. In this way,during the charge re-allocation and integration process, the noise andinterference superimposed on the denoising capacitor may offset thenoise and interference superimposed on the detection capacitor. Whennoise and interference of a driving voltage cause no adverse impact ontothe signal-to-noise ratio of the circuit, a signal-to-noise ratio of afinal output signal is improved. Correspondingly, the designrequirements on other modules may be lowered, the detection difficultyof software and the difficulty of the detection algorithm can belowered, and thus the complexity and overall power consumption of thechip can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

To describe the technical solutions of the present disclosure or theprior art more clearly, hereinafter, drawings that are to be referredfor description of the embodiments or the prior art are brieflydescribed. Apparently, the drawings described hereinafter merelyillustrate some embodiments of the present disclosure. Persons ofordinary skill in the art may also derive other drawings based on thedrawings described herein without any creative effort.

FIG. 1 is a schematic state diagram of a conventional capacitancevariation detection circuit;

FIG. 2 is another schematic state diagram of a conventional capacitancevariation detection circuit;

FIG. 3 is a schematic state diagram of a capacitance variation detectioncircuit according to an embodiment of the present disclosure;

FIG. 4 is a schematic diagram of another capacitance variation detectioncircuit according to an embodiment of the present disclosure;

FIG. 5 is another schematic state diagram of a capacitance variationdetection circuit according to an embodiment of the present disclosure;and

FIG. 6 is a flowchart of a touch detection method according to anembodiment of the present disclosure.

DETAILED DESCRIPTION

To make a person skilled in the art better understand the technicalsolutions of present disclosure, the technical solutions according tothe embodiments of the present disclosure are clearly and completelydescribed with reference to the accompanying drawings of the embodimentsof the present disclosure. Obviously, the embodiments described hereinare merely exemplary ones, but are not all the embodiments. Preferredembodiments are illustrated in the accompanying drawings. The presentdisclosure may be practiced in various ways, and the practice is notlimited to the embodiments described hereinafter. On the contrary, theseembodiments are provided to make the disclosure of the presentdisclosure more thoroughly and completely understood. Based on theembodiments of the present disclosure, all other embodiments derived bypersons of ordinary skill in the art without any creative efforts shallfall within the protection scope of the present disclosure.

Unless otherwise defined, all the technical and scientific terms used inthis specification are the same as those usually understood by personsskilled in the art of the present disclosure. The terms in thespecification of the present disclosure are only used to describe thespecific embodiments, but not to limit the present disclosure. The terms“comprise”, “include” and variations thereof in the specification,claims and accompanying drawings are intended to define a non-exclusivemeaning.

Term “embodiments” in this specification signifies that the specificcharacteristic, structure or feature described with reference to theembodiments may be covered in at least one embodiment of the presentdisclosure. This term, when appears in various positions of thedescription, neither indicates the same embodiment, nor indicates anindependent or optional embodiment that is exclusive of the otherembodiments. A person skilled in the art would implicitly or explicitlyunderstand that the embodiments described in this specification may beincorporated with other embodiments.

In an embodiment of the present disclosure, FIG. 3 illustrates a statediagram of a capacitance variation detection circuit according to thisembodiment.

The circuit includes a sampling module I and an integrating module II.Specifically, as illustrated by two dotted line blocks in FIG. 3, thesampling module I includes a detection capacitor Ctp, a denoisingcapacitor C1 and a plurality of switches, i.e., switches SW1 to SW6 asillustrated in FIG. 3; the integrating module II includes an operationalamplifier SOP1, an integrating capacitor C2 and a switch SW7; anddevices and modules inside the sampling module I and the integratingmodule II may be connected in the following manners:

One terminal of the detection capacitor Ctp is connected to a referenceground, and the other terminal of the detection capacitor Ctp isconnected to a power source Vch via the switch SW1 and is connected toone terminal of the denoising capacitor C1 via a switch SW2 andconnected to a negative input terminal of the operational amplifier SOP1via a switch SW3. One terminal of the denoising capacitor C1 isconnected to the reference ground via a switch SW4. Optionally, asillustrated in FIG. 4, a resistor R1 is connected between the switch SW3and the negative input terminal of the operational amplifier SOP1.

The other terminal of the denoising capacitor C1 is connected to a powersource Vch via a switch SW5, and is connected to the reference groundvia a switch SW6. Optionally, the denoising capacitor C1 is acapacitance-adjustable capacitor. Specifically, the denoising capacitorC1 is adjusted to a suitable capacitance according to the capacitance ofthe detection capacitor Ctp.

Two terminals of the integrating capacitor C2 are connected via a switchSW7.

A positive input terminal of the operational amplifier SOP1 is connectedto a power source Vcm. Optionally, as illustrated in FIG. 4, a resistorR2 is connected between the power source Vcm and the positive inputterminal of the operational amplifier SOP1.

Optionally, the power source Vch is a driving voltage source or adriving current source, and the capacitance variation detection circuitis driven by using the driving voltage source or the driving currentsource.

The capacitance variation detection circuit according to this embodimentof the present disclosure supports two operating states:

at a sampling stage, such as a circuit operating state illustrated inFIG. 3; where in this operating state, the switches SW1, SW4 and SW5 areturned on, whereas the switches SW2, SW3 and SW6 are turned off; and

at an integrating stage, such as a circuit operating state illustratedin FIG. 5; where in this operating state, the switches SW1, SW4 and SW5are turned off, whereas the switches SW2, SW3 and SW6 are turned on.

With the capacitance variation detection circuit according to thisembodiment of the present disclosure, a capacitance variation caused bya touch operation or fingerprint identification may be quickly detected.

In this embodiment, a touch screen is further provided. The touch screenincludes the capacitance variation detection circuit as described in theabove embodiment. The specific composition of the capacitance variationdetection circuit may be referenced to the relevant content disclosed inthe above embodiment, which is not described herein any further.

In an embodiment, as illustrated in FIG. 6, a touch detection method isprovided. The method is based on the capacitance variation detectioncircuit as described in the above embodiment. The method includes:

S1: At a first stage, one terminal of a detection capacitor and oneterminal of a denoising capacitor are connected to a first powersimultaneously, and the other terminal of the detection capacitor andthe other terminal of a denoising capacitor are connected to a referenceground simultaneously.

Specifically, referring to the circuit operating state diagramillustrated in FIG. 3, the switch SW1 between the detection capacitorCtp and the power source Vch is turned on, one terminal of the denoisingcapacitor C1 is connected to the power source Vch, and the otherterminal of the denoising capacitor C1 is connected to the referenceground, that is, the switches SW1, SW4 and SW5 are turned on, whereasthe switches SW2, SW3 and SW6 are turned off.

It may be observed that the power source Vch charges the detectioncapacitor Ctp, and also charges the denoising capacitor C1 configured tooffsetting noise. Since the detection capacitor Ctp and the denoisingcapacitor C1 are simultaneously charged by the power source Vch, thenoise loaded by the power source Vch onto the detection capacitor Ctpand the denoising capacitor C1 has the same phase and amplitude as otherinterference signals.

S2: At a second stage, the detection capacitor is disconnected from thefirst power source, and the detection capacitor is connected to thenegative input terminal of the operation amplifier, the one terminal ofthe denosing capacitor is switched from being connected to the firstpower source at the first stage to being connected to the referenceground, and the other terminal of the denoising capacitor is connectedto the negative input terminal of the operational amplifier.

Specifically, referring to the circuit operating state diagramillustrated in FIG. 5, the switch between the detection capacitor Ctpand the power source Vch is turned off, the negative input terminal ofthe operational amplifier SOP1 is connected, the one terminal of thedenosing capacitor is switched from being connected to the first powersource at the first stage to being connected to the reference ground,and the other terminal of the denoising capacitor C1 is connected to thenegative input terminal of the operational amplifier SOP1. That is, theswitches SW1, SW4 and SW5 are turned off, whereas the switches SW2, SW3and SW6 are turned on.

It may be known that at the second stage, the denoising capacitor C1 isequivalent to being inverted and then connected to the circuit, and thecharges on the detection capacitor Ctp and the denoising capacitor C1are re-allocated, such that an output Vout at an output terminal of theoperational amplifier SOP1 may be calculated according to the law ofcharge conservation:

$\begin{matrix}{V_{out} = {\frac{\left( {V_{ch} - V_{c\; m}} \right)\left( {C_{tp} - C_{1}} \right)}{C_{2}} + V_{c\; m}}} & (1)\end{matrix}$

V_(ch,N) is used to represent the noise or interference loaded onto thepower source Vch, and with respect to V_(ch,N), similarly based on thelaw of charge conservation, an output throughout integration is:

$\begin{matrix}{V_{{out},N} = \frac{V_{{ch},N}\left( {C_{tp} - C_{1}} \right)}{C_{2}}} & (2)\end{matrix}$

It may be observed that since the detection capacitor Ctp and thedenoising capacitor C1 are both charged by the power source Vch, if thevalue of Ctp-C1 in equation (2) is sufficiently small, the noise orinterference output corresponding to V_(ch,N) is weakened or eliminated.

S3: An output result of the operational amplifier is acquired, andwhether there is a touch operation is judged according to the outputresult.

Specifically, the output Vout before variation is compared with theoutput Vout after variation to calculate a variation value of thecapacitor Ctp, such that whether there is a touch operation orfingerprint identification operation may be judged.

Optionally, before judging whether there is a touch operation accordingto the output result of the operational amplifier SOP1, the acquiring anoutput result of the operational amplifier SOP1 specifically includes:repeatedly performing the operations at the first stage and the secondstage according to a predetermined execution count N to obtain theoutput result, where the execution count N is a positive integer. Theoperations may be performed several times or hundreds of times accordingto the specific circuit. Optionally, the predetermined execution countvaries with different detection capacitors. Through sampling integrationfor multiple times, the value of the output result Vout may beamplified, for the convenience of detection judgment. In the aboveembodiment, the switch SW7 may retain a turned-off state. That is, theswitch is not arranged in the circuit.

In another embodiment, optionally, when the operation at the first stageis performed, the switch SW7 between two electrode plates of theintegrating capacitor C2 is turned off to discharge the remainingcharges in the integrating capacitor C2. After the integrating capacitorC2 is discharged, the switch SW7 between the two electrode plates of theintegrating capacitor C2 is turned off again to start a new integrationcycle. In this way, the integration cycle may not be affected due to theremaining charges in the integrating capacitor C2.

Optionally, the capacitance of the denoising capacitor C1 may beadjustable. The method further includes adjusting the capacitance of thedenoising capacitor C1 according to the capacitance of the detectioncapacitor Ctp. A signal-to-noise ratio satisfying the detection need maybe obtained by adjusting the capacitance of the denoising capacitor C1.

With the capacitance variation detection circuit, the touch screen andthe touch detection method according to the embodiments of the presentdisclosure, a denoising capacitor samples noise and interference causedto a detection capacitor, and impacts caused by the noise andinterference are eliminated by means of charge re-allocation.Specifically, while the detection capacitor is being charged, thedenoising capacitor is connected to the same power source, andtherefore, the noise and interference loaded onto the detectioncapacitor and the denoising capacitor have the same phase. In this way,during the charge re-allocation and integration process, the noise andinterference superimposed on the denoising capacitor may offset thenoise and interference superimposed on the detection capacitor. Whennoise and interference of a driving voltage cause no adverse impact ontothe signal-to-noise ratio of the circuit, a signal-to-noise ratio of afinal output signal is improved. Correspondingly, the designrequirements on other modules may be lowered, the detection difficultyof software and the difficulty of the detection algorithm are lowered,and thus the complexity and overall power consumption of the chip arereduced.

Described above are exemplary embodiments of the present disclosure,which are not intended to limit the protection scope of the presentdisclosure. Although the present disclosure is described in detail withreference to the above embodiments, a person skilled in the art wouldstill make modifications to the specific embodiments and the technicalsolutions disclosed therein, or would still make equivalent replacementsto a part of the technical features therein. Any equivalent structuremade based on the specification and accompanying drawings of the presentdisclosure, even if being directly or indirectly applied to some otherrelated technical fields, shall all fall within the protection scope ofthe present disclosure.

What is claimed is:
 1. A capacitance variation detection circuit,configured to detect a capacitance variation of a detection capacitor,comprising: a denoising capacitor, wherein one terminal of the denoisingcapacitor is connected to a first terminal of the detection capacitorvia a second switch and to a reference ground via a third switch, thefirst terminal of the detection capacitor is connected to a first powersource, a second terminal of the detection capacitor is connected to thereference ground, and the other terminal of the denoising capacitor isconnected to the first power source via a fourth switch and to thereference ground via a fifth switch; an operational amplifier, wherein apositive input terminal of the operational amplifier is connected to asecond power source, and a negative input terminal of the operationalamplifier is connected to the first terminal of the detection capacitorvia a first switch; and an integrating capacitor, wherein two terminalsof the integrating capacitor are connected via a sixth switch, and areconnected between the negative input terminal of the operationalamplifier and an output terminal of the operational amplifier.
 2. Thecapacitance variation detection circuit according to claim 1, whereinthe first power source is a driving voltage source or a driving currentsource.
 3. The capacitance variation detection circuit according toclaim 1, wherein the denoising capacitor is a capacitance-adjustablecapacitor.
 4. The capacitance variation detection circuit according toclaim 1, wherein a first resistor is connected between the second switchand the negative input terminal of the operation amplifier.
 5. Thecapacitance variation detection circuit according to claim 4, whereinthe first resistor is connected between the first switch and thenegative input terminal of the operation amplifier.
 6. The capacitancevariation detection circuit according to claim 4, wherein a secondresistor is connected between the second power source and the positiveinput terminal of the operation amplifier.
 7. The capacitance variationdetection circuit according to claim 1, further comprising a seventhswitch, wherein the seventh switch is connected between the first powersource and the detection capacitor.
 8. The capacitance variationdetection circuit according to claim 7, wherein at a sampling stage ofthe capacitance variation detection circuit: the third switch, thefourth switch and the seventh switch are turned on; the first switch,the second switch, the fifth switch are turned off; and the sixth switchis turned on or turned off; and at an integrating stage of thecapacitance variation detection circuit: the third switch, the fourthswitch, the sixth switch and the seventh switch are turned off; and thefirst switch, the second switch, the fifth switch are turned on.
 9. Thecapacitance variation detection circuit according to claim 1, whereinthe detection capacitor and the denoising capacitor are connected to thefirst power simultaneously at a first stage; and the detection capacitoris connected to the negative input terminal of the operation amplifierat a second stage, while the one terminal of the denosing capacitor isconnected to the reference ground and the other terminal of the denosingcapacitor is connected to the negative input terminal of the operationamplifier.
 10. A touch screen, comprising: a detection capacitor,wherein a first terminal of the detection capacitor is connected to afirst power source via a seventh switch, a second terminal of thedetection capacitor is connected to the reference ground; and acapacitance variation detection circuit, configured to detect acapacitance variation of the detection capacitor, wherein thecapacitance variation detection circuit comprises: a denoisingcapacitor, wherein one terminal of the denoising capacitor is connectedto the first terminal of the detection capacitor via a second switch andto a reference ground via a third switch, and the other terminal of thedenoising capacitor is connected to the first power source via a fourthswitch and to the reference ground via a fifth switch; an operationalamplifier, wherein a positive input terminal of the operationalamplifier is connected to a second power source, and a negative inputterminal of the operational amplifier is connected to the first terminalof the detection capacitor via a first switch; and an integratingcapacitor, wherein two terminals of the integrating capacitor areconnected via a sixth switch, and are connected between the negativeinput terminal of the operational amplifier and an output terminal ofthe operational amplifier.
 11. The touch screen according to claim 10,wherein the first power source is a driving voltage source or a drivingcurrent source.
 12. The touch screen according to claim 10, wherein thedenoising capacitor is a capacitance-adjustable capacitor.
 13. The touchscreen according to claim 10, wherein a first resistor is connectedbetween the second switch and the negative input terminal of theoperation amplifier.
 14. The touch screen according to claim 13, whereinthe first resistor is connected between the first switch and thenegative input terminal of the operation amplifier.
 15. The capacitancevariation detection circuit according to claim 14, wherein a secondresistor is connected between the second power source and the positiveinput terminal of the operation amplifier.
 16. The touch screenaccording to claim 10, wherein at a sampling stage of the capacitancevariation detection circuit: the third switch, the fourth switch and theseventh switch are turned on; the first switch, the second switch, thefifth switch are turned off; and the sixth switch is turned on or turnedoff; and at an integrating stage of the capacitance variation detectioncircuit: the third switch, the fourth switch, the sixth switch and theseventh switch are turned off; and the first switch, the second switch,the fifth switch are turned on.
 17. The touch screen according to claim10, wherein the detection capacitor and the denoising capacitor areconnected to the first power simultaneously at a first stage; and thedetection capacitor is connected to the negative input terminal of theoperation amplifier at a second stage, while the one terminal of thedenosing capacitor is connected to the reference ground and the otherterminal of the denosing capacitor is connected to the negative inputterminal of the operation amplifier.
 18. A touch detection method,applied to a touch screen, wherein the method comprises: at a firststage, connecting one terminal of a detection capacitor and one terminalof a denoising capacitor to a first power simultaneously, and connectingthe other terminal of the detection capacitor and the other terminal ofa denoising capacitor to a reference ground simultaneously; at a secondstage, disconnecting the detection capacitor from the first powersource, connecting the detection capacitor to the negative inputterminal of the operation amplifier, switching the one terminal of thedenosing capacitor from being connected to the first power source at thefirst stage to being connected to the reference ground, and connectingthe other terminal of the denoising capacitor to the negative inputterminal of the operational amplifier; and acquiring an output result ofthe operational amplifier, and judging whether there is a touchoperation according to the output result.
 19. The method according toclaim 18, wherein the acquiring an output result of the operationalamplifier comprises: repeatedly performing the first stage and thesecond stage according to a predetermined execution count to obtain theoutput result.
 20. The method according to claim 19, wherein thepredetermined execution count varies with different detectioncapacitors.